The present invention relates to a method and apparatus for generating controlled torques on objects. The invention is particularly useful for generating controlled torques in order to steer objects through a living body for purposes of performing minimally-invasive diagnostic or interventional procedures, and the invention is therefore described below with respect to such an application.
Minimally-invasive diagnostic or interventional procedures require three classes of devicesxe2x80x94viewing devices to provide feed-back to the operator (since direct viewing as in open surgery is not available), operational devices (i.e. tools to perform the task), and controller devices which manipulate or navigate the operational devices. Most commonly, viewing devices are based on optical instrumentation with optic fibers or imaging modalities like X-ray, ultrasound, computerized tomography (CT) or magnetic resonance imaging (MRI). The operational devices vary with the procedurexe2x80x94endoscopes and catheters for diagnostic and interventional procedures; and miniature specialized tools for laparoscopic and other minimally-invasive interventions. The control of the devices is most commonly achieved by mechanical mechanisms. Examples include: 1) endoscopes, which are inserted into a lumen (e.g. the gastro-intestinal tract, the bronchial tree), are navigated by viewing through the endoscopes, and have mechanical control of the tip direction; 2) catheters which are inserted through blood vessels, either veins or arteries, to perform diagnostic procedures (e.g. coronary catheterization) or interventions (e.g. angioplasty of stenosed blood vessels or cardiac valves), and are navigated by mechanical manoeuvres (e.g. combinations of pushing, pulling and twisting of the external portion of the catheter) together with real-time viewing of the blood vessels and the catheters using X-ray imaging; and 3) various rigid devices for cellular aspiration, tissue biopsy, or other diagnostic and interventional procedures, which are inserted with real-time guiding (e.g. by ultrasound) or by stereotaxis guidance.
Computer-assisted stereotaxis is a valuable technique for performing diagnostic and interventional procedures, most typically with the brain. During traditional stereotaxis, the patient wears a special halo-like headframe, and CT or MRI scans are performed to create a three-dimensional computer image that provides the exact location of the target (e.g. tumor) in relation to the headframe. When this technique is used for biopsy or minimally-invasive surgery of the brain, it guides the surgeon in determining where to make a small hole in the skull to reach the target. Newer technology is the frameless technique, using a navigational wand without the headframe (e.g. Nitin Patel and David Sandeman, xe2x80x9cA Simple Trajectory Guidance Device that Assists Freehand and Interactive Image Guided Biopsy of Small Deep Intracranial Targetsxe2x80x9d, Comp Aid Surg 2:186-192, 1997).
Many of the advantages of MRI that make it a powerful clinical imaging tool are also valuable during interventional procedures. The lack of ionizing radiation, and the oblique and multiplanar imaging capabilities, are particularly useful during invasive procedures. The absence of beam-hardening artifacts from bone allows complex approaches to anatomic regions that may be difficult or impossible with other imaging techniques such as conventional CT. Perhaps the greatest advantage of MRI is the superior soft-tissue contrast resolution, which allows early and sensitive detection of tissue changes during interventional procedures. Many experts now consider MRI to be one of the most powerful imaging techniques to guide interventional interstitial procedures, and in some cases even endovascular or endoluminal procedures (Yoshimi Anzai, Rex Hamilton, Shantanu Sinha, Antonio DeSalles, Keith Black, Robert Lufkin, xe2x80x9cInterventional MRI for Head and Neck Cancer and Other Applicationsxe2x80x9d, Advances in Oncology, May 1995, Vol 11 No. 2).
Virtually all current guiding and manipulation methods are based on various mechanical or electro-mechanical modules. For example, steerable catheters use tension wires to bend the tip of the catheter to the desired direction, and typically enable bending in one plane; endoscopes have mechanical control of the tip direction in two orthogonal planes, using two knobs on their control unit; rigid devices are oriented externally before they are inserted into the body to reach the defined target. The major drawback of these mechanisms is their relative complexity and high cost, which typically result with devices for multiple use.
A somewhat different approach to navigation and manipulation is based on magnetic stereotaxis. Current stereotactic procedures with rigid devices, although less invasive than open surgery, may still damage various structures along the path of insertion. The magnetic stereotaxis instrumentation (Stereotaxis Inc., St. Luis, Mo.) is less destructive. According to this technique surgeons insert a magnetic pellet the size of a rice grain into a small hole drilled into the skull of a patient, and the patient""s head is then placed in a housing which contains six superconducting magnets. Using previously recorded MRI or CT images or real-time X-ray imaging as a guide, the surgeon directs the pellet through the brain by adjusting the forces of the various magnets. The pellet could tow a catheter, electrode or other device to the target. However, magnetic stereotaxis cannot be used with real-time MRI because of the MRI scanner""s strong magnetic field, which precludes the use of magnetic objects inside the body during MRI scanning.
From the presented background on current methodologies, one can define the ideal system for minimal invasive procedures: It should provide real-time, 3-dimensional, non-ionizing imaging (like MRI or ultrasound) as feed-back to the user for optimal insertion and intervention; and it should implement flexible, miniaturized devices which can be manoeuvred through an optimal path to minimize damage to healthy tissues and sensitive organs.
One object of the present invention is to provide a method and apparatus for generating controlled torques to be applied to objects, which method and apparatus are particularly useful for maneuvering miniaturized devices through an optimal path in a living body to minimize damage to healthy tissues and sensitive organs.
Another object of the present invention is to provide a method and apparatus to control and manipulate a device inside a living body through the generation of magnetic dipoles in the device which interact with an external magnetic field, like the magnetic field of an MRI system, and thus generate torque or torques for controlling and manipulating the device.
According to one aspect of the present invention, there is provided a method of generating a controlled torque of a desired direction and magnitude in an object within a living body, comprising: producing an external magnetic field of known magnitude and direction within the body; applying to the object a coil assembly including at least three coils whose axes are of known orientation with respect to each other and have components in the three orthogonal planes; and controlling the electrical current through the coils to cause the coil assembly to generate a resultant magnetic dipole interacting with the external magnetic field to produce a torque of the desired direction and magnitude.
According to further features in the preferred embodiment described below, the coils have axes oriented orthogonally with respect to each other; and the external magnetic field is a steady, homogenous magnetic field, particularly the main magnetic field of an MRI (Magnetic Resonance Imaging) system.
MRI is rapidly becoming the preferred methodology for minimal invasive diagnostic and interventional procedures because of its non-invasiveness, high resolution, high contrast between different soft tissues, and absence of shadowing by bones. Recent technological improvements in MRI systems provide rapid scanning sequences, which enable real-time imaging during the procedure, and an open architecture which enables access to the patient. The present invention makes use of a basic, universal component of the MRI systemxe2x80x94the steady, homogenous magnetic field B0, typically generated by a superconducting electromagnetic coil; but the invention may also be applied with other sources of external or internal magnetic fields.
Any magnetic field exerts torque on magnetic dipoles, like the one generated by an electrical current in a closed-loop wire or a coil (Biot-Savart and Ampere Laws). The torque on the coil depends on the relative direction of the dipole with respect to the direction of the magnetic field. With at least three coils, for example three orthogonal coils, a magnetic dipole with any spatial direction can be generated: each coil generates a dipole, which can be represented by a vector, and the combined three coils generate a dipole which is the vectorial sum of the three dipoles.
One can generate such a dipole with any magnitude and direction by controlling the electrical currents through each of (he three individual coils, which determine the magnitude of the dipole in each coil. If the orientation of the three coils in the magnetic field is known, a specific magnetic dipole (i.e. with specific magnitude and direction) can be generated. This controllable dipole interacts with the external magnetic field to generate a controllable torque, namely a torque with a specific magnitude and direction.
The generated torque can be used to bend the tip of a catheter or endoscope and thus to enable the operator to advance the device in the required direction. Furthermore, the torque can be used to operate various devices to perform different activities inside the body, similar to mechanical devices used during laparoscopic procedures. For example, a pliers-like clamping mechanism can be used to hold or release objects inside the body; a miniature cutting device can be used to perform remote surgery; and a miniature stapler-like device can be used to suture structures.
The present invention has significant advantages over existing methodologies. Compared with mechanical devices for navigation and operation of various diagnostic and interventional devices, electromagnetic devices constructed in accordance with the present invention for the same tasks will be smaller, cheaper, and will enable more precise control of the position, direction and operation of the device.